Some Dinosaurs Were Dealt a Slow Death

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Some dinosaur species were declining long before the
150-million-year-long Age of Dinosaurs ended, scientists find.

Apparently large herbivores such as Triceratops and the
duck-billed dinosaurs saw a long-term decline before the
catastrophe, but carnivores and other plant-eaters, such as giant
sauropods, did not, researchers said. Why some dinosaurs were on
their way out while others still thrived just before "the end"
may have to do with their locations — whether they lived in North
America or Asia, for instance.

To explore this question further, vertebrate paleontologist
Stephen Brusatte at the American Museum of Natural History in New
York and his colleagues investigated seven major dinosaur groups
during the end of the Cretaceous, encompassing nearly 150
species. Specifically, they analyzed the variability of the
anatomy and body plans within those groups. Groups that show
increasing diversity might have flourished in their environments
and evolved into more species, while decreasing variability might
be a
warning sign of extinction in the long term.

"People often think of dinosaurs as being monolithic — we say,
'The dinosaurs did this, and the dinosaurs did that,'" said
researcher Richard Butler of Ludwig Maximilian University in
Munich. "But dinosaurs were hugely diverse. There were hundreds
of species living in the Late Cretaceous, and these differed
enormously in diet, shape and size. Different groups were
probably evolving in different ways and the results of our study
show that very clearly."

The scientists found that biodiversity of large herbivores,
including the
duck-billed hadrosaur dinosaurs and horned ceratopsid
dinosaurs such as Triceratops, seemingly experienced a
long-term decline during the last 12 million years of the Age of
Dinosaurs. In contrast, a number of other dinosaurs stayed
relatively stable or even may have slightly increased in
biodiversity, including carnivores such as tyrannosaurs, mid-size
herbivores such as the armored ankylosaurs and bone-headed
pachycephalosaurs, and truly enormous herbivores, such as
sauropods, that gulped their food whole.

The picture of dinosaur biodiversity grows even more complex if
one takes different locations into account. Although hadrosaurs
apparently declined in North America, their diversity was
increasing in Asia during the late Cretaceous. (The Cretaceous
Period, which lasted from about 145 million to 65 million years
ago, was the last part of the Age of Dinosaurs.) [ Dinosaur
Detective: Find Out What You Really Know ]

"Few issues in the history of paleontology have fueled as much
research and popular fascination as the extinction of non-avian
dinosaurs," Brusatte said. "Did sudden volcanic eruptions or an
asteroid impact strike down dinosaurs during their prime? We
found that it was probably much more complex than that, and maybe
not the sudden catastrophe that is often portrayed."

Location matters

A number of factors in North America might have influenced
the evolution of dinosaurs there as compared with other
continents, including mountain formation and extreme fluctuations
in size and sea level of the Western Interior Seaway, a vast
inland sea that divided what is now North America in half.

"The mountain-building and changes in the sea would have meant
the land area in North America was constantly growing and
shrinking, and so it would make sense that animals living on that
land would change in an evolutionary sense as well," Brusatte
told LiveScience. "It also makes sense that you would see
declines in large plant-eaters such as hadrosaurs and ceratopsids
first. They were distant relatives, but ecologically they were
both doing similar things — they were essentially at the bottom
of the food chain, the major dinosaur in terms of the landscape,
much more common than other dinosaurs, so it would make sense
they would be affected first by any change in the environment."

The researchers note that just because some dinosaur groups might
have been in decline before their end "does not automatically
mean that dinosaurs were
doomed to extinction," said researcher Mark Norell, chair of
paleontology at the American Museum of Natural History. "Dinosaur
diversity fluctuated throughout the Mesozoic, and small increases
or decreases between two or three time intervals may not be
noteworthy within the context of the entire 150-million-year
history of the group."

Future research will focus on finding more dinosaurs of this age
in other parts of the world. "That should help make the picture
of the time right before the extinction clearer," Brusatte said.

The scientists detailed their findings online May 1 in the
journal Nature Communications.